You can convert to Roth whenever you want. The conversion is taxable income in whatever year the conversion is performed. A conversion now would therefore be reportable on your 2017 tax return. With a potentially significant amount having been rolled over to an IRA from the 401(k), the conversion may be largely taxable, including only a small portion of your basis in nondeductible contributions. If you are eligible to make a Roth IRA contribution, you would likely be better off do that than making a nondeductible traditional IRA contribution and later converting to Roth.
